Thin film transistors for flexible electronics require low cost high volume manufacturing technologies and materials compatible with roll-to-roll printing. However, many commercially available dielectrics used in fabrication of printable electronics are not ideal for such a purpose.
For example, inorganic dielectrics, such as Al2O3 and other oxides currently used in the flexible electronics industry, need to be deposited in vacuum by either physical vapour deposition or atomic layer deposition. Both processes are not compatible with low cost, high speed roll-to-roll manufacturing of flexible electronics, because after deposition of the inorganic dielectric, a patterning step, such as lithography, has to be performed, which adds complexity and cost to the overall process.
As for commercially available organic dielectrics suitable for the manufacture of thin film transistors, these are usually processed at high temperatures (>120° C.), which presents a problem, since flexible electronics require processing at relatively low temperatures (i.e. room temperature) because some of the materials cannot withstand high processing temperatures. For example, higher temperatures often cause deformations and misalignments in the thin film transistor stack.
Moreover, most polymer dielectrics are not compatible with certain solvents used in the production of flexible electronics. For example, polymethyl methacrylate (PMMA) is often used as a dielectric. However, PMMA has the disadvantage of being soluble in many organic solvents used during processing, which limits the choices of inks and materials that may be used during processing. In addition, PMMA has a relatively low dielectric constant (<3.8).
One possible route to obtain polymer dielectrics insoluble in organic solvents is to cross-link the polymer chains. While some commercially available cross-linkable dielectrics require high baking temperatures, >120° C., the ideal formulation would be processable at room temperature and under open air. In photoinduced free radical polymerization and cross-linking, the reactions occur at room temperature. Generally free radical polymerizations and crosslinking reactions are carried out in the absence of oxygen, which is a known inhibitor. This may be accomplished by removing oxygen from the solution of acrylates, and then performing the polymerization under an inert gas atmosphere. However, these steps are cumbersome and difficult to do in a roll-to-roll printing line.
From the above-description it is clear that there is an opportunity to develop polymer formulations that are more conducive to use in roll-to-roll printing applications, and which may be photopolymerized and cross-linked under open air at room temperature.